Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Generally, transmissions include at least one negative speed ratio which is engaged when the driver selects reverse.
Many automatic transmissions implement a discrete number of different transmission ratios in which each ratio is establish by engaging a particular subset of friction shift elements and/or other shift elements. Shift elements may include devices that couple two rotating elements to one another, commonly called clutches, and devices which couple a rotating element to a stationary element, commonly called brakes. To shift from one speed ratio to another speed ratio, one shift element, called the off-going element, is released and another shift element, called the oncoming element, is engaged. Some types of shift elements, such as dog clutches, have no capability to absorb energy during engagement. When the oncoming element is a shift element of this type, the shafts to be coupled must be at very nearly the same speed before engagement.
When the vehicle is stationary, the gearbox input is also stationary even for very high speed ratios. Since an internal combustion engine cannot generate torque at zero crankshaft speed, a launch device is necessary to permit the engine to rotate and transmit torque to the gearbox input. Many automatic transmission utilize a torque converter having an impeller driven by the engine crankshaft and a turbine driving the gearbox input shaft. Torque is transferred from the impeller to the turbine whenever the impeller rotates faster than the turbine. Torque is transferred in the opposite direction when the turbine rotates faster than the impeller.